This invention relates generally to wood product manufacture, and more particularly to a non-stick belt or platen for a laminating press.
There are a wide variety of products in which a thermosetting polymer, for example, melamine, is laminated to a wood product substrate such as particle board, under controlled conditions of pressure and temperature. The heat necessary for polymeric curing and producing a desired surface texture (glossy, stippled, wood grain, herringbone, etc.) are imparted by metal plates or belts.
Plate presses are much more common than belt-type machines. They typically are single-acting presses in which the laminate is squeezed between textured, heated top and bottom plates backed by heat-conducting pads. The platens may be made of chromium-plated stainless steel, or plated brass. Brass has the advantage of high thermal conductivity, resulting in shorter cycle times, but stainless steel is harder and lasts longer. For wood product panel pressing, such platens are chrome plated, because melamine will stick to either stainless steel or brass.
In a belt-type machine, the laminate is passed between upper and lower stainless steel belts, each of which travel around a pair of heated drums maintained at about 200.degree.-210.degree. C. The heat transferred to melamine-soaked paper by the belt produces cross-linking ("curing") of the polymer at about 160.degree. C. The stainless belts normally used are plated with chromium, or chromium on copper, possibly in varying thickness if a surface texture is desired.
Both belt and plate laminating presses have some problems. Because they are surface plated, present plates or belts cannot be repaired or reworked. Additionally, their surfaces are subject to damage from foreign objects such as nuts, bolts and rocks. In belt applications, which are subject to flexure as the belt passes around its non-circular path, the brittle chromium plating is subject to cracking and spalling after a few hundred thousand flexural cycles, particularly in welded regions of the belt. Also, non-uniform heating and mechanical forces may warp the belt, making it difficult to steer the belt properly around the drums. Attempting to correct this condition by increasing belt tension sometimes results in permanently stretching the belt.